This invention relates to apparatus for probing a microwave circuit.
A known form of apparatus for probing a microwave integrated circuit comprises a dielectric substrate, a coplanar waveguide on a first main surface of the substrate, and a mounting structure to which the substrate is attached. The substrate has an end region that is shaped to permit the end region to be brought into close proximity with an integrated circuit under test, and an opposite edge. The coplanar waveguide comprises a signal conductor extending from the aforesaid edge of the substrate to the end region and two ground conductors extending adjacent the signal conductor along opposite respective edges thereof. At the end region, the signal conductor and the two ground conductors are connected to respective contact bumps. The mounting structure comprises a block of metal having a generally planar surface region to which the substrate is adhesively bonded. Conductive bonding material extends along the aforesaid edge of the substrate and provides electrically conductive connection between the metal block and the ground conductors. The metal block is formed with a bore that opens at the generally planar surface region. A conductive pin extends within the bore, and dielectric material is disposed within the bore and separates the pin from the metal block. The pin and the metal block are connected respectively to the core conductor and the outer conductor of a coaxial waveguide. The pin is connected to the signal conductor of the coplanar waveguide.
This type of probing apparatus is used for probing microwave integrated circuits in wafer form, by positioning the apparatus so that the contact bumps at the end region of the substrate contact respective pads of a circuit under test. The metal block must be quite large in order to allow it to be mounted in a stable fashion. This implies that when multiple contact pads are to be probed simultaneously, so that multiple metal blocks are distributed around the circuit under test, the blocks must be at a substantial distance from the circuit under test. This in turn implies that the length of a substrate, from the edge at which contact is made to the ground conductors to the end region at which the contact bumps are located, is substantial and therefore the length of the ground conductors must be substantial.
The coplanar waveguide and the coaxial waveguide each have a uniform characteristic impedance. The connections between the conductors (both signal and ground) are designed to provide a smooth transition between the two waveguide configurations. For example, the size of the bore in metal block and the dielectric constant of the dielectric material are selected to provide a capacitance between the pin and the metal block sufficient to compensate the inductance of the portion of the pin that projects beyond the planar surface region of the metal block.
The conductive bonding material that connects the metal block to the ground conductors of the coplanar waveguide interposes a finite impedance between the metal block, which is a hard ground, and the ground conductors of the coplanar waveguide. This impedance in the ground path causes delay in propagation of the signal from the contact bumps to the coaxial waveguide, resulting in signal distortion.
The ground conductors of the coaxial waveguide have a finite inductance. It is desirable that the inductance of the ground path between the contact bumps and the metal block be minimized, in order to enable the lumped capacitance that must be provided to compensate for the inductance of the ground path to be minimized. Moreover, it is desirable that losses of RF energy be minimized. Since a major source of losses is attributable to the skin effect in the ground conductors of the coplanar waveguide, and the losses due to the skin effect are a function of the length of the ground conductors, it is desirable that the length of the ground conductors be minimized. The need to reduce the length of the ground conductors is in conflict with the ability to probe multiple contact pads of a circuit simultaneously.